1. Field of the Invention
This invention relates to a method of manufacturing a molded coil for use in an electric apparatus, such as a molded transformer or a reactor.
2. Description of the Prior Art
The casting resin and the internal conductor in a molded coil expand and contract to some extent in accordance with their respective coefficients of thermal expansion as a result of the generation of heat during operation, or the variation in ambient temperature during the interruption of operation. As the casting resin and the internal conductor are joined to each other, the difference in their coefficients of thermal expansion results in development of thermal stress in the resin layer. This thermal stress may be expressed by equation (1), as is generally known: EQU .sigma.=Er.multidot.(.alpha..sub.r -.alpha..sub.c).multidot..DELTA.T (1)
in which .sigma. stands for the thermal stress, E.sub.r stands for the Young's modulus of the casting resin, .alpha..sub.r and .alpha..sub.c stand for the coefficient of thermal expansion of the casting resin and the internal conductor, respectively, and .DELTA.T stands for the temperature difference. If the thermal stress exceeds the tensile strength of the resin layer, the resin layer is likely to crack. If the resin layer has cracked, the cracked portion develops a corona discharge, and absorbs moisture, resulting in an unavoidable reduction in the insulation performance of the resin layer.
In order to lower the thermal stress, therefore, it has been proposed to reduce the difference between the coefficients of thermal expansion of the casting resin and the conductor (.alpha..sub.r -.alpha..sub.c ). For example, it has been found effective to use an aluminum conductor instead of a copper one. The casting resin, aluminum and copper have a coefficient of thermal expansion of 31.0 .mu./.degree.C., 23.0 .mu./.degree.C. and 16.6 .mu./.degree.C., respectively. The difference in coefficient of thermal expansion is 8.0 .mu./.degree.C. in case an aluminum conductor is used, while it is 14.4 .mu./.degree.C. in the case of a copper conductor. The use of an aluminum conductor can reduce the thermal stress in the resin layer to about a half of that which develops in the event a copper conductor is used.
The aluminum conductor is, however, lower in conductivity than the copper one. It is necessary to lower the current density of a winding extremely, and the coil requires an increased volume, and a greater amount of resin. Moreover, the use of aluminum does not mean the elimination of the thermal stress on the resin layer; there is still every likelihood that the resin layer may crack.